Someone told me that if your parent marries a gentile, you are halachically not supposed to sit shiva for them when they pass away. This doesn't sound like it could be true.
Can you please clarify?
Nitei Gavriel Aveilus 12:2 quoting Yoreh Deah 345:5 says that one who is a Mumar = convert to another religion, Moseir, or Apikores = Heretic, there is no Aveilus for them unless they did Teshuva (repented), or are considered a Tinok Shenishbo.
If they were murdered or killed then Aveilus applies, as their death atones. Rama Yore Deah 340:5.